The present invention relates generally to the measurement of parameters of human blood and more particularly to an apparatus and method for measuring blood parameters such as oxygen saturation level and hematocrit in a living patient.
A continuous and adequate supply of oxygen is essential to health. Diminution or interruption of the oxygen supply, even for a brief moment, can result in unconsciousness, injury to vital organs, and death. Thus it is often medically necessary to monitor the amount of oxygen being received by the body, especially during critical surgical procedures in which there is an inherent risk of interruption of the oxygen supply.
Monitoring the oxygen being received by the body is accomplished by measuring the oxygen content of the blood, because it is the blood that carries oxygen from the lungs to all parts of the body. Oxygen in the blood is actually carried by hemoglobin, a protein in red blood cells. The blood comes into contact with free oxygen in the lungs, and the hemoglobin combines with molecules of the free oxygen. As the blood flows it carries the hemoglobin and the oxygen throughout the body. The oxygen is released by the hemoglobin to the muscles and other bodily organs where it is used.
Hemoglobin which is combined with oxygen is called oxyhemoglobin; hemoglobin which is not combined with oxygen is known as deoxyhemoglobin or reduced hemoglobin. At any given time and location in the body, a certain fraction s of the hemoglobin in the blood is oxyhemoglobin and the remaining fraction 1-s is reduced hemoglobin. The oxygen content of a sample of blood is usually expressed as a saturation level percentage, which is the ratio of oxyhemoglobin to total hemoglobin. If the fraction s of oxyhemoglobin in a certain sample of blood is 25%, that sample is said to be 25% saturated with oxygen or to have a blood oxygen level of 25%.
The total amount of oxygen in the blood is determined not only by the oxygen saturation level but also by the total amount of hemoglobin present in the red blood cells. Thus, it is important to know the hemoglobin concentration (hematocrit) h, as well as the saturation level in order to determine how much oxygen is being carried by the blood.
It is a relatively straight-forward procedure to measure the oxygen saturation level s and hematocrit h of a sample of blood which has been removed from a patient, for example by means of a hypodermic needle, and transported in a test tube or the like to a medical laboratory. Known methods of laboratory analysis provide highly accurate results, and for many medical purposes these methods are entirely sufficient. However, the process of removing blood from a patient and analyzing it in a laboratory takes time, and if it is necessary to monitor the amount of oxygen in the blood on a real-time basis, as is often the case during surgery, such a procedure is not adequate.
In addition, it is sometimes necessary to monitor the amount of oxygen being received by a specific organ such as the heart. This need may arise, for example, during certain kinds of coronary surgery. Analysis of a sample of blood which has been taken from a bodily location such as an artery of an arm or a leg does not provide the required information respecting the amount of oxygen being provided to the heart or some other internal organ.
Both oxyhemoglobin and reduced hemoglobin absorb light, but certain wavelengths are absorbed more readily by the one than by the other. This difference is easily seen with the unaided eye in that freshly oxygenated blood (in which most of the hemoglobin is oxyhemoglobin) is bright red in color whereas blood from which the oxygen has been removed (most of the hemoglobin being reduced hemoglobin) is darker and has a bluish hue. Accordingly, various devices and methods have been proposed for determining the amount of oxygen in the blood by measuring the attenuation of a beam of light as it passes through a sample of blood.
U.S. Pat. No. 3,638,640, issued to Robert F. Shaw on Feb. 1, 1972, typifies a class of such proposals in which light is directed through an ear lobe or the like. Shaw seeks to determine the oxygen level of the blood in the ear lobe by measuring the attenuation of the light at each of a plurality of wavelengths as the light passes through the ear lobe.
A somewhat similar device is disclosed in U.S. Pat. No. 4,621,643, issued to William New, Jr., et al. on Nov. 11, 1986. This device utilizes the principles of the Shaw invention but with the addition of an encoding resistor which indicates the wavelengths of the light sources in the device to facilitate the calculation of the oxygen level of the blood.
U.S. Pat. No. 3,799,672, issued to Gerald G. Vurek on Mar. 26, 1974 illustrates another device which is based on the principle of attenuation of light passing through a sample of blood. Instead of directing the light through an ear lobe or other bodily appendage, Vurek passes the light through a sample of blood circulating from a patient through a plastic tube outside the body.
In another class of such devices, two optical fibers are introduced into a blood vessel. Light passes into the bloodstream through one of the fibers; some of this light is reflected back through the other fiber, and the oxygen level of the blood is determined by measuring the intensity of the reflected light at each of a plurality of wavelengths. U.S. Pat. No. 4,623,248 issued to John M. Sperinde on Nov. 18, 1986 is illustrative of such devices.
It will be apparent from the foregoing that there remains a need for an accurate way to measure not only the oxygen saturation level but also the hematocrit of human blood on a real-time basis and at a desired location in the body of a patient.